1. Field of the Invention
The present invention relates to laminated safety glass glazing units of the type that are normally used in aircraft. A typical aircraft window is a laminated unit comprising a cover plate, an interlayer and a pressure plate. The cover plate is usually an outer glass sheet having a transparent electroconductive coating on its inner surface interconnecting a pair of bus bars mounted along longitudinal edges of said inner surface with a lead wire connecting each bus bar to an electric terminal, which, in turn, is connected to a source of potential through a switch in order to provide a potential difference between the bus bars. Electrical energy applied across the bus bars is transmitted through the transparent electroconductive coating to heat the cover plate sufficiently to remove fog or even ice that forms on the outer surface of the cover plate, and, hence, the electrical circuit that includes the bus bars and the transparent electroconductive coating serves as a defogging or deicing device. A typical aircraft panel of this type is similar to that of U.S. Pat. No. 3,081,205 to Norman Shorr.
Other aircraft panels dispense with the coating on a glass sheet surface and use a network of fine wires embedded throughout substantially the entire extent of the interlayer to form an electroconductive heating circuit connected between spaced bus bars. U.S. Pat. No. 2,813,960 to Arthur Egle and Walter Bethge shows this type of laminated window.
The pressure plate is usually an inner glass sheet that is usually both larger and thicker than the cover plate, with its marginal portion beyond the margin of the cover plate held in pressurized relation against the frame in which the glazing unit is installed. The interlayer is integral with both the cover plate and the pressure plate and is usually larger than the pressure plate and is composed of a thermoplastic resin. The interlayer is usually reinforced by a metal frame embedded therein that extends around the marginal portion of the unit. A reinforcing frame surrounds three sides of the pressure plate and the fourth side of the latter is received directly in the aircraft frame. Bolt holes in the metal frame and in the reinforcing frame are aligned with holes along the marginal portion of the frame in which the unit is installed in an aircraft. Thus, aircraft windows or units are attached to and held against the frame of the aircraft about their marginal portion only.
Aircraft windows are normally exposed to severe temperature variations in use. At high altitudes, the outer surface of the unit is exposed to low temperature and low pressure, whereas the coated surface of the cover plate is at the elevated temperature of the coating and the inner surface of the pressure plate, that is, the surface of the pressure plate exposed to the interior of the aircraft, is exposed to the pressurized condition and the moderate temperature within the aircraft. The interlayer has a higher coefficient of thermal expansion than either glass sheet of the unit. Hence, the interlayer tends to contract more rapidly than the glass during exposure to the low temperatures at high altitudes when the aircraft is in flight. This difference in contraction causes the glass to be stressed as the plastic interlayer tends to shrink away therefrom, thereby causing stresses in the glass sheets, particularly in the vicinity of the bus bars bonded to the face plate along the inner side thereof and within the extended portion of the pressure plate beyond the edge of the face plate. Furthermore, additional stresses arise from the pressure differential between the pressurized cabin and the low pressure outside the aircraft, from impact with objects in flight, from other local high stress points resulting from mechanical or thermal stresses due to non-uniform application of adhesive used to bond adjacent portions of the panel to one another and to the frame, from voids in the adhesive, from locally applied loads, from non-uniformity of electroconductive coating of the defogging device when the latter is operated, from thermal shock, and from thermal gradients due to speed, altitude or the manner of operating the aircraft.
Whatever the causes of the stresses, the fact that the interlayer tends to contract more rapidly than the glass enhances these local stresses and causes the glass to be additionally stressed, particularly around the marginal portion thereof. This difference in thermal contraction results in the plastic interlayer tending to shrink away from the marginal portion of the glass and results in chips formed on the glass surface. This tendency to chip is known in the art as "cold chipping". Sometimes these stresses are so great as to cause complete fracturing of the glass.
"Cold chipping" is especially inherent in laminated windows having rigid transparent panels of unequal size. This inherent characteristic has been attributed to the lack of balance on opposite sides of the plastic interlayer due to the difference in dimensions between the cover plate and the pressure plate.
Another problem experienced in laminated glazing units exposed to extremely low temperatures is the tendency of the polyvinyl butyral interlayer to crack along its edges, particularly along the edges that interconnect the edges of the unit along which bus bars are applied.
2. Description of the Prior Art
The problem of "cold chipping" has been recognized and many solutions have been proposed in the prior art. For example, U.S. Pat. Nos. 2,584,859 to Gaiser and 2,650,976 to R. A. Gaiser et al. disclose applying a special tape or strip material as a parting material at the marginal portion of the unit, either at the marginal portion of a glass-plastic interface, but particularly over the area of the electrode on the glass to avoid or minimize the possibility of electrode failure in such units due to the differential contractions between the plastic interlayer and the portion of the glass sheet on which the bus bar is mounted.
Applying a strip or tape of parting material along the edges of the unit interconnecting the edges along which the bus bars are applied does not reduce the effect of cracking of the polyvinyl butyral interlayer when the unit is cooled to low temperatures when in use.
U.S. Pat. No. 2,758,042 to Richard F. Raymond and Emil A. Fusca discloses incorporating a balancing strip in the interlayer of a laminated unit on the side opposite the marginal area of the larger pressure plate that extends beyond the margin of the cover plate to reduce cold chipping. U.S. Pat. No. 2,991,207 to Philip A. Miller discloses slitting the interlayer in spaced relation to the interfacial surface between the glass and the interlayer as another solution to the cold chipping problem.
Another solution proposed to eliminate cold chipping and cracking of the interlayer along its edges was to enclose the unit within an electroconductive wire and selectively heat the margin of the unit by radiation from the heated wire.
Such proposals provided some improvement in cold chipping and/or reduced cracking of the interlayer margin somewhat. However, they still left something to be desired in the reduction of "cold chipping" of the glass and cracking of the plastic interlayer margin, particularly at the lower temperatures experienced by aircraft windows flying at higher elevations than previously.